You might've met this giant armoured man early on, in the Forest of Fallen Giants, when a humongous bird drops him on top of a roof. There's a good chance he killed you. The trick here is roll dodging his advances. Circle strafe is unreliable because he attacks very quickly, and with large sweeping strikes.
Dodge, attack, block, repeat--but watch out for when his sword glows blue, as that attack curses you, reducing your max HP until you use a human effigy to restore it.
For more help be sure to check out our complete Dark Souls 2 Boss guide.